The present invention relates to valves operated by a motorized servo actuator for controlling the amount of opening and the closing of the valve. Such arrangements of controlling valve opening by a servo motor have been employed for precision control of the amount of opening of a butterfly-type water valve utilized for controlling flow of engine coolant to a vehicle passenger compartment heat exchanger. Typically, butterfly valves employed for the aforesaid application utilize an eliptically shaped disc as the valve member, which is welded to a shaft extending through the valve housing for rotary connection to the servo actuator.
Butterfly water valves of the aforesaid type typically employ a rubber seal ring molded about the periphery of the valve member to ensure positive sealing of the valve member against a valve seat provided peripherally about the valve member in the valve housing. In this regard, it has been found necessary for the servo actuator to provide a positive closing torque on the butterfly to maintain the rubber seal in contact with the valve seat to prevent leakage of the valve in the closed position. One typical construction widely employed for vehicle passenger compartment heat exchanger valves uses a weldment to attach the valve member to its pivot shaft. In order for a motorized rotary servo actuator to provide a positive closing torque on the valve shaft when the valve is in the closed position, it is necessary that the servo actuator output shaft continue rotating a minor amount after the valve has reached the closed position.
Typically, servo actuators employed for vehicle passenger compartment heat exchanger water valves utilize a small subfractional horsepower motor driving through a high ratio speed reducer to provide high torque and low rotational speed at the servo actuator output shaft. It is not uncommon for such a servo actuator to utilize a small twelve volt DC motor having a shaft speed of 3,000 rpm to drive a servo actuator employing an eight hundred to one (800:1) speed reduction ratio at its output shaft. Thus, despite the low motor torque, on the order of one or two inch ounces of torque, it will be apparent that the output torque of the servo actuator is substantial and capable of causing damage to the butterfly water valve. Experience has shown that where the servo actuator attempts to rotate the butterfly shaft after the valve member has reached the closed position, the output torque of the servo actuator is sufficient to break the welded attachment of the butterfly valve disc to its shaft within the valve.
Accordingly, it has long been desired to provide a way or means of limiting the torque applied by a servo actuator for controlling a rotary actuated valve; and, in particular, to provide sufficient control of the torque applied by the servo to the valve to prevent breaking of the connection between the valve member and its actuation shaft.